The present invention generally relates to a method and system for automatic electrical sintering and, more particularly, to such method and system for automatically loading powder material into a sintering mold and subsequently effecting electrical sintering to the powder material in the sintering mold in a continuous fabrication process.
There have been provided various electrical sintering technologies; however, they require relatively long time to complete the sintering process, so there has not been proposed an idea of a continuous fabrication process for obtaining sintered products, including the steps of: loading an amount of powder material into a sintering mold; effecting electrical sintering to the powder material in the sintering mold; and removing the sintered product from the sintering mold. As the result, there has not been provided a system for carrying out electrical sintering process in a fully automated manner and in a continuous fabrication process.
Recently, many improvements has been made in electrical sintering methods. For example, Pulsed Current Energizing Sintering (or Pulsed Electric Current Sintering) method using a pulsed current and including Spark-Plasma Sintering, Electric-Discharge Sintering and Plasma-Activated Sintering methods proposed by the applicant of this application has been improved. According to the improved Pulsed Current Energizing Sintering, sintering time is drastically shortened. Such shorter sintering time provides the possibility of realizing a continuous fabrication process for obtaining sintered products, including the above mentioned steps.
In addition, by virtue of newer electrical sintering methods, such as those mentioned above, materials that were difficult to bond together through any older methods can be now bonded together with ease into a unitary sintered product. Examples of such materials are: a stainless steel vs. copper; a ceramic vs. a metal; etc. Such a unitary sintered product of two different powder materials may be fabricated to have two-layered structure composed of two layers bonded together and each made of a pure powder material; however, the characteristics of such a sintered product can be improved by adding at least one middle layer to create such multi-layered structure in that the middle layer is made of a mixture of the two powder materials. Further, such multi-layered structure may be also used advantageously for a sintered product including three or more layers made of respective powder materials which are identical in composition and differ from one another only in particle size, wherein the powder materials for the layers have their particle sizes gradually increasing from the layer on one side of the product toward the other side. Such a sintered product may have gradient functionality (i.e., the gradual variation in properties of the sintered product from one side of the product to the other) so as to achieve more improved characteristics. In order to fabricate a sintered product having gradient functionality, it is required to load different powder materials, which differ from one another in at least one of properties including component(s) of powder material, percentages of components, particle size and particle shape, into a sintering mold so as to form corresponding powder layers of desired thickness"" with precision. However, there has not been proposed a system for automatically loading different powder materials into a sintering mold in the form of a plurality of layers including respective layers of the different powder materials and subsequently effecting electrical sintering to the powder material in the sintering mold.
In view of the foregoing, it is an object of the resent invention to provide a method and system for automatically loading powder material into a sintering mold and subsequently effecting electrical sintering to the powder material in the sintering mold in a continuous fabrication process.
It is another object of the present invention to provide a method and system for automatically loading different powder materials, which differ from one another in at least one of properties including component(s) of powder material, percentages of components, particle size and particle shape, into a sintering mold in the form of a plurality of layers including respective layers of the different powder materials and subsequently effecting electrical sintering to the powder materials in the sintering mold in a continuous fabrication process.
It is a further object of the present invention to provide a method and system for automatically loading powder material into a sintering mold and subsequently effecting electrical sintering to the powder material in the sintering mold, in which high-quality sintered-products may be fabricated in an automated manner and in a continuous fabrication process, by pressing at a desired pressure the powder material filled into the sintering mold.
It is a still further object of the present invention to provide a method and system for automatically loading powder material into a sintering mold and subsequently effecting electrical sintering to the powder material in the sintering mold, in which high-quality sintered products may be fabricated in an automated manner and in a continuous fabrication process, by carrying out pre-heating, sintering and cooling steps within individual, substantially gas-tight chambers for the respective steps isolated from the environment and by individually controlling atmospheres in the chambers.
In accordance with an aspect of the present invention, there is provided a method of automatically loading powder material into a sintering mold and subsequently effecting electrical material to the powder compact in the sintering mold, the method comprising the steps of: fitting a lower press core into a bore formed in the sintering mold, while allowing displacement of the lower press core in the bore; filling different powder materials, which differ from one another in at least one of properties including component(s) of powder material, percentages of components, particle size and particle shape, into the bore to form respective powder layers of the different powder materials in the bore, while displacing the lower press core downward in the bore sequentially and in a stepwise manner, such that the position of a top surface of the lower press core from a top surface of the sintering mold is adjusted depending on the thickness of the respective layers of the different powder materials being filled into the bore; preliminarily pressing at a desired pressure the powder layers of the different powder materials; displacing the pressed powder layers together with the lower press core relative to the sintering mold so as to position the powder layers to a desired position in the sintering mold; fitting an upper press core into the bore of the sintering mold and onto the powder layers; pre-heating to a desired temperature the sintering mold with the powder layers formed therein and with the upper press core fitted therein; and effecting electrical sintering to the powder layers. The last step is carried out by: operating upper and lower conductive members so as to press at a desired pressure the upper press core and the lower press core fitted in the sintering mold having been pre-heated; and simultaneously applying sintering current to the powder layers through the upper and lower conductive members.
In a preferred embodiment of the present invention, the pressing step may be carried out either each time when one of the different powder materials is filled into the sintering mold to form one of the layers, or each time when successive two or more of the different powder materials are filled into the sintering mold to form two or more of the layers. Further, the method may further comprise the step of effecting forced cooling to the sintering mold subsequent to the electrical sintering step.
In another preferred embodiment of the present invention, the pre-heating step may comprise applying pre-heating current to the sintering mold. Also, the sintering current applying step may comprise: causing the lower conductive member to come into contact with only a bottom surface of the lower press core; and applying DC pulsed current to the powder compact through the upper and lower conductive members. Alternatively, the sintering current applying step may comprise: causing the lower conductive member to come into contact with a bottom surface of the lower press core and a bottom surface of the sintering mold; and applying DC pulsed current to the powder compact through the upper and lower conductive members.
In accordance with another aspect of the present invention, there is provided an automatic electrical sintering system for automatically loading powder material into a sintering mold and subsequently effecting electrical sintering to the powder material in the sintering mold, comprising: an automatic powder material loading apparatus for automatically loading a desired amount of powder material into the sintering mold and pressing the powder material: a pre-heating unit disposed along a transportation path of the sintering mold and downstream of the automatic powder material loading apparatus, for pre-heating the sintering mold; a sintering unit including conductive members capable of contact with a pair of press cores fitted in the sintering mold pre-heated by the pre-heating unit, with the powder material being held between the press cores, the sintering unit applying sintering current to the powder material through the plunger conductive members to effect electrical sintering to the powder material in the sintering mold; and a conveyor for conveying through the sintering unit the sintering mold sent from the automatic powder material loading apparatus. Further, the sintering unit includes a substantially gas-tight chamber.
In a preferred embodiment of the present invention, the pre-heating unit may be disposed in the chamber of the sintering unit, and the chamber of the sintering unit may have an atmosphere therein which is controllable to be selectively one of atmospheres including a pre-heating atmosphere and a sintering atmosphere. Alternatively, the pre-heating unit may include its own chamber, and the chamber of the sintering unit and the chamber of the pre-heating unit may have respective atmospheres therein which are individually controllable. Further, the automatic electrical sintering system may further comprise a cooling unit for effecting forced cooling to the sintering mold having been subjected to sintering process, wherein the cooling unit includes its own chamber.
In another preferred embodiment of the present invention, each of the chambers may include an entrance and an exit for the sintering mold to be conveyed therethrough into and out of that chamber; wherein each of the entrance and the exit is provided with a gate for isolating the inside of that chamber from the outside of that chamber. Further, the automatic powder material loading apparatus may serve to fill different powder materials, which differ from one another in at least one of properties including component(s) of powder material, percentages of components, particle size and particle shape, into the bore of the sintering mold to form respective layers of the different powder materials one on another in the sintering mold. In addition, the automatic powder material loading apparatus may comprise: a conveyor system for conveying the sintering mold; a powder filling mechanism for filling an amount of powder material into the sintering mold at a powder filling position defined along a transportation path of the sintering mold conveyed by the conveyor system; and a press unit for pressing at a desired pressure the amount of powder material in the sintering mold; wherein a plurality of the powder filling mechanisms are provided along the transportation path, each having the powder filling position. Alternatively, the automatic powder material loading apparatus may comprise: a conveyor system for conveying the sintering mold; a powder filling mechanism for filling an amount of powder material into the sintering mold at a powder filling position defined along a transportation path of the sintering mold conveyed by the conveyor system; and a press unit for pressing at a desired pressure the amount of powder material in the sintering mold; wherein the powder filling mechanism comprises: a rotary table capable of indexing movement about an axis; and a plurality of hoppers provided on the rotary table at circumferentially spaced positions with respect to the axis of the rotary table. The plurality of hoppers stores the different powder materials therein, respectively, and is capable of individual movement relative to the rotary table. One of the plurality of hoppers is brought to the powder filling position by indexing movement of the rotary table.
In a further preferred embodiment of the present invention, the pre-heating unit may comprise a pair of electrical contacts capable of selective contact with outer side surface of the sintering mold, wherein pre-heating current is applied to the sintering mold through the electrical contacts. Also, the pair of press cores may include an upper press core and a lower press core, wherein the pair of conductive members may include an upper conductive member and a lower conductive member, and wherein the lower conductive member may be capable of contact with only the lower press core fitted in the sintering mold. Alternatively, the pair of press cores may include an upper press core and a lower press core, wherein the pair of conductive members may include an upper conductive member and a lower Conductive member, and wherein the lower conductive member may be capable of contact with a bottom surface of the sintering mold and a bottom surface of the lower press core fitted in the sintering mold. In addition, the automatic electrical sintering system may further comprise a pushing-out unit for pushing out and removing the upper press core, the lower press core and a sintered product at once from the sintering mold having been subjected to sintering process.
In accordance with a further aspect of the present invention, there is provided an automatic electrical sintering system for automatically loading powder material into a sintering mold and subsequently effecting electrical sintering to the powder material in the sintering mold, comprising: an automatic powder material loading apparatus for automatically loading a desired amount of powder material into the sintering mold and pressing the powder material; a sintering unit disposed along a transportation path of the sintering mold and downstream of the automatic powder material loading apparatus and including conductive members capable of contact with a pair of press cores fitted in the sintering mold, with the powder material being held between the press cores, the sintering unit applying sintering current to the powder compact through the conductive members to effect electrical sintering to the powder compact in the sintering mold; a conveyor for conveying through the sintering unit the sintering mold sent from the automatic powder material loading apparatus; a sintering power supply for providing sintering current; and a pre-heating power supply for providing pre-heating current. The conductive members are capable of selective electrical connection to the sintering power supply and the pre-heating power supply, so that the pre-heating current and the sintering current is selectively applicable to the sintering mold through the conductive members. Further, the sintering unit includes a substantially gas-tight chamber having an atmosphere therein which is controllable to be selectively one of atmospheres including a pre-heating atmosphere and a sintering atmosphere.